Vector inductor having multiple mutually coupled metalization layers providing high quality factor

ABSTRACT

An inductor component includes a plurality of conductive elements, each formed as an individual patch of conductive material, with the conductive elements arranged in a vertical stack and tightly coupled to one another. Dielectric is disposed between more adjacent conductive elements, the dielectric has a permittivity and is sufficiently thin so as to provide a mutual inductance factor of at least one-half or greater between adjacent ones of the conductive elements. The dielectric is typically thinner than the adjacent conductors.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of the filing dates of twoco-pending U.S. Provisional Patent Applications entitled “TunablePassive Filter Components”, Ser. No. 61/828,107 filed May 28, 2013, and“Signal Handling Apparatus for Radio Frequency Circuits”, Ser. No.61/857,446 filed Jul. 23, 2013, the entire contents of each of which arehereby incorporated by reference.

BACKGROUND

1. Technical Field

This patent application relates to inductors, and more specifically to avector inductor that provides very high quality factor in a small formfactor.

2. Background Information

An inductor is a passive, two-terminal electrical component whichtemporarily stores electrical energy when a current passes through it.When the current flowing through an inductor changes over time, theresulting time varying magnetic field induces a voltage. An inductor maybe characterized by its inductance, the ratio of this voltage to therate of change of the current.

Inductors are commonly used in many different types of alternatingcurrent (AC) circuits, and especially radio frequency (RF) circuits.Inductors are combined with capacitors and other components to implementfilters, resonators, tuners, and other functions. Electronic devicessuch as smart phones, tablets, laptop computers, and the like are nowtypically expected to use many different radio communication protocolsand operate over a wide variety of frequencies, while at the same timebeing as small and inexpensive as possible. Inductor design becomes acritical aspect of achieving these goals.

SUMMARY

Problem Description

The quality, or Q, factor is a dimensionless parameter thatcharacterizes an inductor in terms of the ratio between the inductanceand the resistance of the component. In RF design, Q is commonlyconsidered to characterize a resonant circuit's bandwidth relative toits center frequency. Higher Q indicates a lower rate of energy lossrelative to the stored energy. Resonators with higher Q factors aretherefore desired for implementations where bandwidth relative to centerfrequency must be increased. The need for high Q is thereforecharacteristic of present day communication devices, which must handlehigher and higher data rates. A high Q tuned circuit also providesgreater selectivity; that is, it does a better job of filtering outsignals from adjacent channels. High Q oscillators also resonate withina smaller range of frequencies and are therefore more stable.

An additional concern with component design is the physical space thatit occupies. Any approach that can increase a given amount of inductanceavailable in a certain amount of circuit area, while also improving Q,would generally be preferred.

If a set of inductors is needed for a particular integrated circuit(IC)-based design, the use of discrete inductors that are separate fromthe ICs may give rise to implementation problems due to the need to makemany interconnections between the set of inductors and the othercomponents. This situation has motivated many circuit designers in thedirection of integrating as many of the needed inductors as possibleinto the semiconductor device itself.

However, the integration of an inductor into an IC creates otherproblems due to an inductor's inherent magnetic properties. IC inductorscan be implemented using a conductor that adopts a “coil” or “spiral”shape as used in classic discrete inductor components. Because the pathfor current flow in such structures is necessarily curved or angled, theinduced magnetic fields tend to force electrons along the shortestpossible path (that is, along the innermost edges) of the spiralconductive path. This in turn limits the ability to improve theinductor's Q with increasing frequency.

As a result, it is generally believed that one must increase the size ofthe conductive path must be increased, or the magnetic coupling betweenadjacent turns, to provide increased Q.

Spiral shaped inductors are therefore believed to be less than ideal forproviding high Q where the inductor must be as small as possible.

Inductors can also be implemented with active circuit componentsincluding transistors. But active circuit based inductors bring otherchallenges in terms of linearity.

SUMMARY OF PREFERRED SOLUTION(S)

The above-mentioned and other related problems motivate aspects of thepresent invention, a vector inductor component that exhibits very high Qin a small form factor that is easily incorporated into IC-based andprinted circuit board designs.

In one arrangement, the vector inductor includes a plurality ofconductive elements, each formed as an individual strip or patch ofconductive material. The conductive elements are arranged in a verticalstack and tightly coupled to one another.

A dielectric is provided between each pair of adjacent conductiveelements in the stack. The dielectric has a permittivity and issufficiently thin so as to provide a mutual inductance factor of atleast one-half or greater, and preferably approaches 0.9 or higherbetween adjacent conductive elements. The high mutual conductance may berealized by constructing the dielectric to be very thin, typically atleast thinner than the adjacent conductors.

The dielectric disposed between the two or more conductive elements mayexhibit a dielectric loss tangent (T_(and)) much less than 1.

The conductors may be connected to one another in a case where theinductor is to serve as a discrete component. Where the inductor is tobe included as part of a parallel or series LC resonant circuit, theinductor may be directly connected to a capacitor via only a topconductive element and a bottom conductive element in the verticalstack.

In one configuration the inductor comprises multiple subassemblies, witheach subassembly including (i) a given one of the dielectric layersprovided by a printed circuit board substrate, and (ii) two conductorsas a metal strip disposed on either side of the is substrate. Theinductor is then formed of multiple subassemblies attached to oneanother via adhesive layers.

The conductors are generally a rectangular strip of metal having atleast two parallel side walls extending from the input end to the outputend to encourage maximum current flow, avoiding curved or angular paths.This ensures that the conductive field path is as straight as possiblefrom an input end to an output end.

BRIEF DESCRIPTION OF THE DRAWINGS

The detailed description below refers to the accompanying drawings, ofwhich:

FIG. 1 is a cross section view of a vector inductor formed of stackedconductors and dielectric layers;

FIG. 2 is a more detailed cross section view of a portion of the vectorinductor;

FIGS. 3A and 3B are top views of two different shapes for theconductors;

FIG. 4 is a more detailed cross section view of a specific embodiment ofthe vector inductor formed of multiple printed circuit boardsubassemblies adhered together;

FIGS. 5A and 5B show modeled inductance and quality factor of the vectorinductor for 16 and 32 stacked elements at 1 GigaHertz (GHz);

FIG. 6 is an isometric view of a vector inductor component;

FIGS. 7A and 7B, respectively, show one possible way to connect thevector inductor in a parallel resonant circuit and series resonantcircuit;

FIG. 8A and 8B show another way to connect the vector inductor inparallel and series resonant circuits;

FIG. 9 is a cross sectional view of a series resonant circuitimplementation of the vector inductor using a printed circuit boardsubstrate and integrated circuit capacitors; and

FIG. 10 is a top level view of an example Chebyshev filter implementedon a printed circuit board using the vector inductor and integratedcircuit capacitors.

DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT

Briefly, the preferred design for a vector inductor uses tightlycoupled, layered sets of conductive patches formed on and/or within aprinted circuit board substrate. The tightly coupled conductors exhibita high mutual inductance factor, at least one-half or preferably even0.9 or higher. In one example embodiment, N mutually coupled inductorsof inductance L with this very tight coupling are fit into an area ofsize 1/N as compared to the size occupied by one uncoupled inductor (ofvalue N*L). This results in a total reduction factor of N² in size foran inductor of inductance L. For N=16, the reduction in size istherefore 256 times smaller than an uncoupled, non-layered inductor.

FIG. 1 is a high level cross section view of one such vector inductor100 formed from multiple, closely spaced patches of conductive material,referred to as the conductors 102 herein. The N conductors 102 may beformed as separate copper layers disposed on and/or within a printedcircuit board substrate. The conductors 102 are aligned with one anothervertically, and spaced apart from one another by multiple dielectriclayers 104. For a vector inductor of N conductors 102, there would beN−1 dielectric layers 104.

The dielectric layers 104 may include any suitable dielectric materialand/or an adhesive layer such as epoxy. In an implementation where theconductors are mechanically suspended at the ends, the dielectric mayeven be air.

The layer thicknesses in FIG. 1 are not shown to scale. To encouragemutual coupling, the thickness of the dielectric layers 104 is typicallyless than the thickness of the conductors 102. In one exampleimplementation the conductor 102 thickness (or height H1) might be about0.33 mils for a quarter-ounce copper patch conductor (0.67 forhalf-ounce copper) but the dielectric 104 thickness might be only 0.1mils. In general, the thickness of the various dielectric layers 104will be the same, although it is possible that some of the dielectriclayers are thicker or thinner than others. Quarter-ounce and half-ouncerefer to the industry standard terminology for metal thickness inprinted circuit board structures.

FIG. 2 shows a more detailed view of one subassembly or section 105 ofthe vector inductor 100, including two adjacent conductors 102 and thedielectric 104 disposed between them. The dielectric 104 has a relativepermittivity ε_(r). It can be shown that the following equation modelsthe resulting relationship of voltage and current:

$V_{1} = {{{L\frac{\frac{i}{2}}{t}} + {M\frac{\frac{i}{2}}{t}}} = {\left( {\frac{L}{2} + \frac{M}{2}} \right)\frac{i}{t}}}$

where L is the inductance of each conductor 102, i is the total currentflowing through the section (such that each conductor 102 carries acurrent of i/2) and we can conclude that:

$V_{1} = {{\left( {\frac{L}{2} + \frac{kL}{2}} \right)\frac{i}{t}} \approx {L\frac{i}{t}\mspace{14mu} {for}\mspace{14mu} k} \approx 1}$

where V₁ is the voltage across the inductor structure section 105, and Mis a mutual inductance factor given by

M=k°{square root over (L ₁ L ₂)}=kL because L₁=L₂=L

where L₁ is the inductance of a first layer, and L₂ is the inductance ofthe second layer.

Therefore, this relation will hold true when the mutual inductance isrelatively high, such that the mutual inductance factor k is at least0.5 and preferably approaches 0.90 or higher.

It should be noted that in comparing the closely coupled inductor pairarchitecture of FIG. 2 to a simple metal strip, the real resistance ofthe inductor is halved, while the total inductance has not changed. Theresult is that the quality factor Q is doubled while the totalinductance remains at approximately L. for a given area. For a singlepair of conductors 102 as shown in FIG. 2, a Q of about 150 is possible.

The material chosen for dielectric 104 disposed between each conductiveelement 102 is such that it exhibits a dielectric loss tangent (T_(and))much less than 1, typically approaching something less than or equal to2e⁻⁵.

The conductors 102 may assume various shapes; again, what is importantis that the conductors 102 are tightly coupled to one another. FIG. 3Ais a top view of one possible implementation of the conductors 102.Here, the conductor 102 is shaped as an elongated rectangle or “strip”of metal. This rectangular strip shape provides the straightest possiblepath for electric field propagation by eliminating any curves or angles.This in turn, maximizes the quality factor for a given configuration.However, other shapes for conductors 102 may also be sufficient to meetthe requirements herein of increased Q. One such possible shape, shownin FIG. 3B, still has a still generally a rectangular main conductiveportion 155, but now having stub sections 157-1, 157-2 on each opposingend. The stub sections 157-1, 157-2 can assist with impedance matchingto adjacent components and/or circuit connections. What is important isto avoid forcing current to flow along a curved propagation pathsthrough the conductor 102, as well as sharp angles in the sidewalls ofany shapes that deviate from the rectangular as much as possible.

A “skin effect” of radio frequency signals propagating via a conductorsuch as a conductive patch 102 causes currents to generally flow on ornear the surface or edges, rather than through the entire thickness ofthe conductor 102. Increasing the thickness of the conductor 102 thuswill not have any appreciable affect on the amount of current carried,or the resistance experienced by the signal propagating through theconductor. This skin effect thus normally limits the ability to increasethe Q and the total inductance in an inductor 102 formed from strips ofconductive material.

However, the inductor pair configuration of FIG. 2 can be extended to amultiple layer vector inductor configuration shown in FIG. 4. Here, anumber, P, of closely coupled inductor pairs or subassemblies 212-1,212-2, . . . , 212-g, . . . , 212-P are stacked together. As with theembodiments of FIGS. 1 and 2, an example inductor element 212-g isformed as a pair of conductive material patches 220-1, 220-2 disposed oneither side of a dielectric 222. Here the dielectric 222 may be formedof the “A-stage” material” of a printed circuit board substrate with theconductors 220-1, 220-2 being copper patches disposed on the respectivetop and bottom sides thereof. The resulting P subassembly constructionsare arranged vertically with respect to one another and adhered to oneanother using “B-stage” material such as an epoxy adhesive 223.

Stacking multiple inductor pairs 212 in this way to form the vectorinductor 100 forces at least some of the currents to flow though theconductors 220 in the middle of the structure in addition to the skineffect on the outermost conductor layers 228-1, 228-2. This improves theoverall conductivity of the vector inductor 100 as compared to a singlesolid conductor of the same dimension.

An adhesive layer 223 is disposed between adjacent ones of the inductorpairs 212; the adhesive 223 is chosen to be relatively thin and have arelatively low static relative permittivity (dielectric constant) ε_(r)so that a given inductor pair 212-g will exhibit tight coupling to itsneighboring inductor pair located immediately above (inductor pair212-g−1) and below (inductor pair 212-g+1).

Mutual coupling of the overall vector inductor structure is determinedby the distance between the layers and the dielectric constant of thematerials disposed between the conductors. FIG. 4 shows some typicaldimensions. For an internal conductive layer 220 thickness (or height)of approximately 0.66 mils (16.74 μm) and dielectric substrate layers222 of approximately 0.315 mils (8 μm), one would prefer to have anε_(r) of the dielectric substrate of about 3.5 and an ε_(r) of theadhesive layers 223 of about 2.7 (if the adhesive 223 is 0.3 mils (7.62μm) thick). Again, the total dielectric layer thickness is less than thethickness of the conductive layers.

The outermost conductors 228-1, 228-2 may preferably be somewhat thickerthan that of the internal conductive layers 220—here the outerconductors may be 2.7 mils (67.54 μm) thick.

It is preferred that each conductor 220 has the same size and shape asthe adjacent conductors 102 (and indeed all other internal conductors220) in the stack that make up the vector inductor structure. However,variations in the size and shape of the individual conductors would notdepart from the spirit of the design.

The stacked inductor design of FIG. 4 provides important advantages overother approaches. Normally, a structure that includes P independentinductors of value L would consume a space that is P times larger thanthe space consumed by the single inductor L. However, in the case of themutually coupled vector inductors of FIG. 4, the P mutually coupledinductors of size L, provided with very tight coupling, only requires aspace of size 1/P, as compared to the space that would be occupied by asingle uncoupled inductor (of value P*L). The total reduction in size isthus P² where N is the number of inductor pairs. Thus if P equals 16,the corresponding reduction in size is 256 times smaller than the caseof the single inductor.

Tightly coupled vector inductors with mutual inductance of 0.95 orhigher shown herein in tend to provide great improvement in theavailable Q factor, achieving a Q of 200 or more.

FIGS. 5A and 5B, respectively, show modeled inductance and qualityfactor provided at an operating frequency of 1 GHz for differentconductive patch widths (in mils) and for two different numbers ofinductor pairs (P=16 and P=32). The model assumed that a 250 mil thickair column is provided adjacent the top and bottom outer conductorlayers 228-1, 228-2.

Curve 502 in FIG. 5A shows modeled inductance varying as a function ofthe width of rectangular conductor strips 220 for the number of layers,P=16, and curve 504 is a similar plot of inductance for P=32. Curve 512in FIG. 5B shows quality factor, Q, varying as a function of the widthof conductor strips 220 for P=16, and curve 514 is variance of Q forP=32.

Consideration can also given to how the vector inductor 100 is ideallyconfigured to connect to other components to make up RF circuits ofvarious types.

FIG. 6 is an implementation of a vector inductor 100 intended forpackaging as a separate or standalone discrete component. As seen inthis isometric view (where the vertical scale is somewhat exaggeratedfrom the actual scale) the conductors 102 stacked above one another andclosely spaced apart by dielectric layers 104 are again seen as in theprior embodiments. Conductive sidewalls 108 are disposed between theconductors 102 and/or along the sides of another structure that supportsconductors 102. These additional connections provided by the conductivesidewalls 108 may further assist with encouraging mutual inductancebetween conductors 102. Here an input terminal 118 and output terminal119 are connected adjacent the bottom-most one of the conductors 102.

In order to maintain overall compact size, certain designs are preferredfor a vector inductor that is to be incorporated into a series orparallel resonant circuit. As understood by those of skill in the art, aresonant circuit may implement a filter that typically includes severalinductors and capacitors, with the number of inductors and capacitors inthe filter and their specific interconnection depends upon the type offiltering desired {band pass, low pass, high pass, band stop, etc.} andalso depending upon the number of poles and zeros desired for such afilter. The discussion below is not concerned with that aspect of filterdesign, but rather the physical configuration and electrical connectionof each individual inductor and capacitor component.

FIG. 7A is a schematic diagram of one such possible implementation of aparallel LC resonant circuit 702. Shown are the vector inductor 100including conductors 102 and dielectric layers 104; the inductor 100again includes a lower most conductor 228-1 and uppermost conductor228-2. The capacitor C is provided by a pair of capacitor elements 704-1and 704-2, each of capacitance C/2, with each capacitor respectivelyconnected in parallel with a respective one of the bottom conductor228-1 and top conductor 228-2. Note that the innermost conductors 102 ofvector inductor 100 are not connected to one another in thisimplementation, nor are they connected to one another. It is thoughtthat this configuration may provide the highest possible Q.

FIG. 7B is an implementation for a series LC circuit that is similar tothe parallel LC circuit of FIG. 7A. Again, the capacitors 706-1, 706-2are only connected to the bottommost 228-1 and topmost 228-2 conductors.The capacitors 706-1 and 706-2 may each have one-half the capacitance Cdesired in the series LC circuit.

Also possible are implementations for a parallel LC circuit, in the caseof FIG. 8A and a series LC circuit in the case of FIG. 8B. Thedifference over FIGS. 7A and 7B is that the vector inductors 100 areimplemented using conductive sidewalls 108. While this configuration maynot provide the optimum performance characteristics, it may still beacceptable in some applications to make construction easier.

FIG. 9 is a more detailed cross-sectional view of a series resonantcircuit 900 consisting of a single vector inductor 100 and pair ofcapacitances C1 and C2. This series resonant circuit 900 is constructedaccording to the principles shown in FIG. 7B but now shown in greaterdetail with integrated circuit (IC) chip type capacitors 706-1-1,706-1-2 providing capacitance C1 and capacitors 706-2-1, 706-2-2providing capacitance C2. This layout is particularly adaptable forimplementation of the vector inductor 100 in a multi-layer printedcircuit board (PCB) substrate 770 (FIG. 7 b) which may be a FaradFlex,or other suitable multi-layer substrates. The capacitors 706-1 and 706-2may be IC chip components mounted on top of the PCB substrate 750 as“flip chips”. One preferred implementation of the capacitor chips 706uses the CMOS capacitor array architecture described in the patentapplications incorporated by reference above.

FIG. 10 is a top level view of a more complex filter design using thesame principals of FIG. 9. Here a set of chip capacitors C1, C2, C3, C4,C5, C6 are connected with a set of inductors I1, 12, 13, 14 to provide aChebychev filter. Each of inductors I1, 12, 13, 14 may be implementedwithin a PCB substrate as a vector inductor 100. Each of the inductorsmay have a different shape to realize a different inductance to realizethe desired filter response.

While various embodiments of the invention have now been particularlyshown in the drawings and described in the text above, it will beunderstood by those skilled in the art that various changes in form anddetails may be made therein without departing from the scope of theinvention. It is intended, therefore, that the invention be limited onlyby the claims that follow.

What is claimed is:
 1. An inductor apparatus comprising: a plurality ofconductive elements, each formed as an individual patch of conductivematerial, the conductive elements arranged in a vertical stack withrespect to one another; and a dielectric disposed between at least twoor more adjacent conductive elements, the dielectric being sufficientlythin so as to provide a mutual inductance factor of at least one-half(½) or greater between adjacent ones of the conductive elements.
 2. Theapparatus of claim 1 wherein a relative permittivity of the dielectricdisposed between the two or more conductive elements exhibits adielectric loss tangent (Tand) much less than
 1. 3. The apparatus ofclaim 1 wherein the conductive elements are each electrically connectedto two or more adjacent conductive elements.
 4. The apparatus of claim 1wherein the inductor further comprises part of a parallel resonantcircuit wherein the inductor is directly connected to a first capacitorvia only a top conductive element and a second capacitor is directlyconnected via only a bottom conductive element in the vertical stack,and wherein other conductive elements disposed between the bottom andtop conductive layer are not connected to the first or second capacitorvia a direct conductor path.
 5. The apparatus of claim 4 wherein two ormore of the other conductive elements are connected to one another. 6.The apparatus of claim 1 wherein the inductor further comprises part ofa series resonant circuit wherein the inductor is directly connected toa first capacitor via only a top conductive element and a bottomconductive element is directly connected to only a second capacitor inthe vertical stack, and wherein other conductive elements disposedbetween the bottom and top conductive layer are not connected to thefirst or second capacitor via a direct conductor path.
 7. The apparatusof claim 6 wherein two or more of the other conductive elements areconnected to one another.
 8. The apparatus of claim 1 wherein theinductor comprises a subassembly including (i) a given one of thedielectric layers as a printed circuit board substrate, and (ii) twoconductors as a metal disposed on either side of the substrate.
 9. Theapparatus of claim 8 wherein the printed circuit board substrate has anembedded capacitor layer.
 10. The apparatus of claim 8 wherein theinductor comprises multiple subassemblies attached to one another withan adhesive layer.
 11. An inductor apparatus comprising: a plurality ofconductors, each formed as an individual patch of conductive material,the conductors arranged vertically with respect to one another to form aconductor stack; and a thin dielectric disposed between two or moreadjacent conductive elements, the dielectric having a thickness lessthan a thickness of the immediate adjacent conductive elements.
 12. Theapparatus of claim 11 wherein the conductors are formed from a generallyrectangular strip of metal having at least two parallel side wallsextending from the input end to the output end.
 13. The apparatus ofclaim 11 wherein the have a shape such that a conductive field path isstraight from an input end to an output end.
 14. The apparatus of claim11 wherein the conductors have a thickness in a range of from about 0.33mils to 0.7 mils.
 15. The apparatus of claim 11 wherein the dielectriclayers have a thickness in a range of from 0.3 to 0.315 mils.
 16. Theapparatus of claim 11 wherein at least one of an uppermost andbottommost conductor of the stack is thicker than inner conductors ofthe stack.
 17. The apparatus of claim 11 wherien the conductors have awidth in a range of from 40 to 80 mils.